低能隙高分子及金屬高分子材料於總體異質接面太陽能電池之合成與應用

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National Chiao Tung University

Department of Materials Science and Engineering

PhD Thesis

Synthesis of Low Band-Gap Polymers and

Metallo-Polymers for Bulk Heterojunction Solar Cells

低能隙高分子及金屬高分子材料於總體異質接面太陽

能電池之合成與應用

Harihara Padhy (哈瑞)

Advisor: Hong-Cheu Lin, Ph.D. (林宏洲教授)

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Synthesis of Low Band-Gap Polymers and

Metallo-Polymers for Bulk Heterojunction Solar Cells

低能隙高分子及金屬高分子材料於總體異質接面太陽

能電池之合成與應用

Student: Harihara Padhy

(哈瑞)

Advisor: Hong-Cheu Lin, Ph.D.

(林宏洲教授)

A Thesis submitted to

Department of Materials Science and Engineering

College of Engineering

National Chiao Tung University

In partial fulfillment of the requirement for the degree of

Doctor of Philosophy

In materials science and engineering

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For my dear daughter

“Akankshya”

“If we did all the things we are capable of doing, we would astound ourselves.” Thomas A. Edison

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Abstract

The main objective of this dissertation is to study the performance of polymer

bulk heterojunction solar cell involving conjugated donor-acceptor

polymers/metallo-polymers as electron donors. In the introduction of this thesis, we

gave an explanation on the historical evolution of polymer solar cells, and

summarized the literature in the recent years.

In the second chapter, we describe the design, synthesis, and polymer solar cells

(PSC) fabrication of a series of soluble donor-acceptor conjugated polymers

comprising of phenothiazine donor and various benzodiazole acceptors (i.e.,

benzothiadiazole, benzoselenodiazole, and benzoxadiazole) sandwiched between

hexyl-thiophene. These low band-gap (LBG) polymers demonstrated broad

absorption in the region of 300-750 nm with optical band gaps of 1.80-1.93 eV. Both

highest occupied molecular orbital (HOMO) (-5.38 to -5.47 eV) and lowest

unoccupied molecular orbital (LUMO) (-3.47 to -3.60 eV) energy levels of the LBG

polymers were within the desirable range of ideal energy levels. The best performance

of the PSC device was obtained by using one of the polymers containing

benzothiadiazole acceptor at the core and [6,6]-phenyl-C71-butyric acid methyl ester

(PC71BM) in the weight ratio of 1:4, and a PCE value of 1.20%, an open-circuit

voltage (Voc) value of 0.75 V, a short-circuit current (Jsc) value of 4.60 mA/cm2, and a

fill factor (FF) value of 35.0% were achieved.

In the third chapter, we describe the design, synthesis, and characterization of

-cyano-thiophenevinylene-substituted polymers containing cyclopentadithiophene and dithienosilole units. The effects of the bridged atoms (C and Si) and

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and photovoltaic properties were investigated. Both LBG polymers had broad

absorption spectra with ideal HOMO (ca. -5.30 eV) and LUMO (ca. -3.60 eV) levels

and possessed hole mobilities as high as 9.82  10-4 cm2/Vs. The PSC device based on one of the polymers containing dithienosilole moiety with PC71BM (1:2 w/w)

exhibited a best power conversion efficiency of 2.25% under AM 1.5, 100 mW/cm2. In the fourth chapter, synthesis, and characterization of a series of π-conjugated bis-terpyridyl ligands bearing various benzodiazole cores and their corresponding

main-chain RuII metallo-polymers were described. The effects of electron donor and acceptor interactions on their thermal, optical, electrochemical, and photovoltaic

properties were investigated. Due to the broad sensitization areas of the

metallo-polymers, their BHJ solar cell devices containing [6,6]-phenyl C61 butyric

acid methyl ester (PC61BM) as an electron acceptor exhibited a high short-circuit

current (Jsc). An optimum PVC device based on the blended polymer with PCBM in

1:1 (w/w) achieved the maximum power conversion efficiency (PCE) value up to 0.45

%, with Voc = 0.61 V, Jsc = 2.18 mA/cm2, and FF = 34.1 % (under AM 1.5 G 100

mW/cm2), which demonstrated a novel family of conjugated polyelectrolytes with the highest PCE value comparable with BHJ solar cells fabricated from ionic

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摘要

本論文的主要目的是研究具有電子予體及受體的聚合物及含金屬聚合物，在異質結太陽能電池中的性質研究。論文首先介紹，聚合物太陽能電池的發展歷程，並做個總結。在第二章中，描述了一系列聚合物設計概念、合成方法及應用在聚合物太陽能電池（PSC）中的效果，這些聚合物主要是由可溶性的phenothiazine做為電子予體，和各種 benzodiazole (如:benzothiadiazole，benzoselenodiazole和benzoxadiazole) 做為電子受體，以及夾在中間的hexyl-thiophene所組成。這些低能隙（LBG型）聚合物具有廣泛的吸收範圍(300-750 nm)，光學能隙在1.80-1.93 eV之間，且 HOMO（-5.38至-5.47 eV）和LUMO（-3.47至-3.60 eV）能階均在LBG聚合物的

理想範圍內。在PSC裝置應用中，以benzothiadiazole為分子內電子受體的聚合物，以重量比1:4混合PC71BM ( [6,6]-phenyl-C71-butyric acid methyl ester)時，所得光電

轉換效率1.20％，為所有聚合物中最佳，其開路電壓0.75 V、短路電流值4.60 mA/cm2時、填充因子35.0％。在第三章中，我們描述一系列含cyclopentadithiophene和dithienosilole的β -cyano-thiophenevinylene-substituted聚合物的設計概念，合成方法和性質分析。並探討橋接原子（C和Si）和cyano-vinylene基團在熱，光，電化學，電荷傳輸和光伏效應等性質上造成的影響。此類LBG聚合物具有廣闊的吸收光譜和理想的 HOMO（約 -5.30 eV）和LUMO（約 -3.60 eV）的能階，並具有相當高的hole

mobility (9.82×10-4 cm2/Vs)。PSC裝置應用中，一含有dithienosilole之聚合物，混合 PC71BM（1:2 w/w）表現出最佳的功率轉換效率為 2.25％，光源為ＡＭ 1.5

下，100 mW/cm2_。

在第四章中，描述合成一系列以benzodiazole為核心的bisterpyridine，利用螯

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以及熱，光，電和光伏效應等性質。由於其廣泛的吸收光譜範圍金屬，嘗試應用於BHJ太陽能電池設備中。混合PC61BM（1:1 w/w）作為電子受體表現出高短路 電流（Jsc）。所得的最大光電轉換效率（PCE）高達 0.45％，開路電壓 = 0.61 eV，短路電流 = 2.18 mA/cm2_{，FF = 34.1％（AM 1.5，100 mW/cm}2_{）。因此證明這種} 新穎的共軛高分子電解質，相較普遍的polythioene，同樣可在ＢＨＪ裝置中，作為供應電子的主動層。

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Acknowledgements

First and foremost I want to thank my advisor Prof. Hong-Cheu Lin. It has been

an honor to be his first foreign Ph.D. student. I appreciate all his contributions of time,

patience, motivation, immense knowledge ideas, and funding to make my Ph.D.

experience productive and stimulating. The joy and enthusiasm he has for his research

was contagious and motivational for me, even during tough times in the Ph.D. pursuit.

I could not have imagined having a better advisor and mentor for my Ph.D study.

I owe my deepest gratitude also to Dr. Chih-Wei Chu and his group members for

their support in a number of ways including device fabrications and characterizations.

My thanks and appreciation goes to my thesis committee members, for their

encouragement, insightful comments, and suggestions. I owe my deepest gratitude to

Prof Kung-Hwa Wei (Chairman), and all professor and secretaries of Department of

Materials Science and Engineering, NCTU for their support.

The members of our group have contributed immensely to my personal and

professional time at NCTU. The group has been a source of friendships as well as

good advice and collaboration. I am especially grateful to all group members

including Rajan, Hsuan-Chih, Duryodhan, Wei-Hong, Dhananjaya, Yen-Hsing,

Hsiao-Ping, Mutheya, Rudrakanta, Ashutosh, Ramesh, Murali, Raju, I-Hung, Han,

Chong-Lun, Chung-Ji, Ming-shaw, Shin-Chieh, Chia-Lin, Kuan-Ying and Li-Han. I

would like to show my gratitude to Dr. Kartik for introducing me with my advisor and

also for his valuable help during my tough time.

I would like to thank prof. Sangram Mudali (Director), Prof. Ajit Panda ( Dean),

Prof. Arun Padhy and all my collogues at National Institute of Science and

Technology, Berhampur, Orissa, India for their encouragement and help in many

directions.

Last but not least, I would like to thank my family for all their love and

encouragement. For my parents who raised me with a love of science and supported

me in all my pursuits. And most of all for my loving, supportive, encouraging, and

patient wife Manoswini Nayak and daughter Akankshya whose faithful support

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List of Figures and Schemes

Figure 1.1 Terrestrial cell efficiencies measured under the global AM1.5

spectrum. ... 2

Figure 1.2 Schematic device structure for bulk heterojunction solar cells. ... 3 Figure 1.3 General mechanism for photoenergy conversion in excitonic solar

cells. ... 4

Figure 1.4 Current (voltage) characteristics of a typical organic diode shown

together with the metal-insulator-metal (MIM) picture for the

characteristic points. (a) Short circuit condition. (b) Open circuit

condition. (c) Forward bias. (d) Reverse bias. ... 8

Figure 1.5 Four device architectures of conjugated polymer-based photovoltaic

cells: (a) single-layer PV cell; (b) bilayer PV cell; (c) disordered bulk

heterojunction; (d) ordered bulk heterojunction. ... 9

Figure 1.6 Absorption coefficients of films of commonly used materials in

comparison with the standard AM 1.5 terrestrial solar spectrum. ... 13

Figure 1.7 Resonance structures in benzo-bis-thiadiazole. ... 15 Figure 1.8 Alternating donor–acceptor units lower the effective band gap by

orbital mixing. ... 15

Figure 1.9 Example of organic semiconductors used in polymer solar cells. ... 17 Figure 1.10 Nonexhaustive survey of reports focusing on photovoltaic devices

based on P3HT:PCBM blends. ... 19

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Figure 1.12 General structure of the fluorene copolymers with a variety of

acceptors.General structure of the fluorene copolymers with a variety

of acceptors. ... 21

Figure 1.13 General structure of the phenothiazine containing copolymers. ... 22 Figure 1.14 Structure of the carbazole copolymers with a variety of acceptors. ... 22 Figure 1.15 Structure of the diketopyrrolopyrrole containing copolymers with a

variety of donors. ... 23

Figure 1.16 Structure of the polymers containing donating moieties from the

2,2‟-bithiophene unit covalently bridged with an atom at 3,3‟-position. ... 24

Figure 1.17 Chemical structure of PBDTTT–E, PBDTTT–C and PBDTTT–CF. .. 24 Figure 1.18 Structure of the conjugated polyelectrolytes used in solar cells. ... 25 Figure 1.19 Nonexhaustive list of investigation techniques required for an extended

characterization of active materials for polymer solar cells. ... 26

Figure 2.1 Synthetic Routes of Monomers. ... 41 Figure 2.2 Synthetic Routes of Polymers (PP6DHTBT, PP6DHTBSe, and

PP6DHTBX). ... 42

Figure 2.3 1H NMR spectra of polymers in CDCl3. ... 43

Figure 2.4 TGA thermograms of polymers. ... 44 Figure 2.5 Normalised UV-vis spectra of polymers in (a) dilute chlorobenzene

solutions and (b) solid films, respectively. ... 46

Figure 2.6 Cyclic voltammograms of polymers ... 49 Figure 2.7 Current-voltage curves of polymer solar cells using polymer:PCBM

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Figure 2.8 (a) Absorbance spectra of PP6DHTBT:PC71BM thin films measured

from the solar cell devices by using an ITO/PEDOT substrate as a

reference. (b) External quantum efficiency (EQE) of

PP6DHTBT:PC71BM solar cells. ... 55

Figure 2.9 AFM images of PP6DHTBT: PC71BM blend films. (a) 1:1 (w/w), (b)

1:3 (w/w), and (c) 1:4 (w/w) ratios. ... 56

Figure 3.1 Synthetic route for monomers and polymers. ... 70 Figure 3.2 TGA measurements of polymers at a heating rate of 10°C/min. ... 71 Figure 3.3 Normalized absorption spectra of polymers in dilute chloroform

solutions (10-6 M) and solid films. ... 72

Figure 3.4 Cyclic voltammograms of polymers in solid films at a scan rate of 100

mV/s. ... 74

Figure 3.5 (a) Current-voltage curves of polymer solar cells using polymer:PCBM

blends under the illumination of AM 1.5G, 100 mW/cm2. (b) EQE curves of the PSC devices based on polymers/PC61BM (1:1, w/w). .. 76

Figure 3.6 AFM images of (a) CPDT-CN: PC61BM 1:1 (w/w) and (b) DTS-CN:

PC61BM 1:1 (w/w). ... 79

Figure 4.1 Synthetic Route for Bis-terpyridyl Ligands (M1-M3) and

RuII-containing Metallo-Polymers (P1-P3)………..91

Figure 4.2 1H NMR spectra (aromatic region) of bis-terpyridyl ligands M1-M3 (in

CDCl3) and metallo-polymers P1-P3 (in DMSO-d6). ... 93

Figure 4.3 TGA thermograms of bis-terpyridyl ligands (M1-M3) and

metallo-polymers (P1-P3) at a heating rate of 10°C/min under nitrogen.

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Figure 4.4 UV-vis absorption spectra acquired upon the titration of (a) M1, (b)

M2, and (c) M3 (in 2:8 v/v CH3CN:CHCl3) with Zn(OAc)2 (in EtOH).

The insets show the normalized absorption at 325 nm as a function of

Zn2+:M2-M3, respectively. ... 95

Figure 4.5 Normalized UV-vis spectra of bis-terpyridyl ligands (M1-M3) and

metallo-polymers (P1-P3). In (a) dilute (10-6 M) solutions and (b) solid films, respectively. ... 99

Figure 4.6 Cyclic voltammograms of bis-terpyridyl ligands (M1-M3) and

metallo-polymers (P1-P3) at a scan rate of 100 mV/s. ... 101

Figure 4.7 Current-voltage curves of BHJ solar cells using blended films of

M1-M3 or P1-P3:PCBM (1:1 w/w) under the illumination of AM 1.5G,

100 mW/cm2. ... 104

Figure 4.8 EQE curves of the PSC devices based on polymers P1-P3: PCBM (1:1

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List of Tables

Table 2.1 Molecular Weights and Thermal Properties of Polymers ... 44

Table 2.2 Optical Properties of Polymers ... 45

Table 2.3 Electrochemical Properties of Polymers ... 49

Table 2.4 Photovoltaic Properties of Polymer Solar Cell Devices with the

Configuration of ITO/PEDOT:PSS/Polymer:PCBM/Ca/Al ... 52

Table 2.5 Annealing Effects on Polymer Solar Cell Device Containing

PP6DHTBT:PC71BM (1:4 wt%) ... 58

Table 3.1 Molecular Weights and Thermal Properties of Polymers ... 72

Table 3.2 Optical and electrochemical properties of Polymers. ... 74

Table 3.3 Photovoltaic properties of PSC devices with the configuration of

ITO/PEDOT:PSS/Polymer:PCBM/Ca/Al. ... 78

Table 4.1 Optical, Thermal, and Viscosity Properties of Bis-terpyridyl Ligands

(M1-M3) and Metallo-Polymers (P1-P3) ... 97

Table 4.2 Electrochemical Properties of Bis-terpyridyl Ligands (M1-M3) and

Metallo-Polymers (P1-P3) ... 101

Table 4.3 Photovoltaic Properties of BHJ Solar Cell Device with a Configuration

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Chapter 1. Introduction

1.1 History of Photovoltaic Cells

The predictable exhaustion of fossil energy resources and the increasing pressure

generated by environmental concerns and climate change have triggered an

intensification of research on the most clean sustainable energy sources, in particular

on the photovoltaic conversion of solar energy. The „photovoltaic effect‟ is the mechanism in which a solar cell converts photons from the solar light into electricity.

While the photovoltaic effect was first observed in 1839 in an electrochemical process

by French physicist Alexander-Edmond Becquerel,1 the first well-performing solid-state solar cell was built by Charles Fritts in 1883. He coated the semiconductor

Selenium with an very thin layer of gold to form a junction that had an efficiency of

1%.2 Modern generation of solar cells was born in 1953 when at Bell Laboratories (New Jersey, USA) the first silicon solar cell was developed with a power conversion

efficiency of 6%.3 After that, many different technologies and materials were developed in order to improve the performance of the device and lower their

production cost. In order to achieve this goal, organic materials provide us with a

variety of possibilities. Especially semiconducting polymers combine the favorable

opto-electronic properties of organic materials, such as high absorption coefficients,

with the excellent solution processability onto a flexible substrate using simple and

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Figure 1.1 Terrestrial cell efficiencies measured under the global AM1.5 spectrum.5

The current status of solar cells is shown Figure 1.1. In more than 20 years since

the seminal work of Tang,6 organic solar cells have undergone a gradual evolution that has led to energy conversion efficiencies of about 7.9%. Solution-processed

organic solar cells were first reported in 1995,7 where, the best efficiency reported at

that time barely reached values higher than 1%, but now could be achieved easily efficiencies beyond 5% today. To attain efficiencies approaching 10 % for the commercialization in large area, much effort is required to understand the fundamental electronic interactions as well as the complex interplay of device architecture, morphology, processing, and the fundamental electronic processes.

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1.2 General Principle of Polymer Solar Cells

1.2.1 Organic Solar Cells on the Basis of Mechanistic Principles

The architecture of a typical Organic Solar Cells is sketched in Figure 1.2. The

core of the cell is the photoactive layer, which is generally composed by a p-type

electron-donor compound (D) and an n-type electron-acceptor compound (A). Both A

and D are organic π-conjugated materials, and either one or the other (or both) is a polymer. The photoactive layer, typically around 100−200 nm in thickness, is

interposed between the electrodes; additional layers of electron or hole transporting

materials can be present.

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Figure 1.3 General mechanism for photoenergy conversion in excitonic solar cells.9

Figure 1.3 illustrates the mechanism by which light energy is converted into

electrical energy in the devices. The energy conversion process has four fundamental

steps in the commonly accepted mechanism:10

1. Absorption of light and generation of excitons: Photoexcitation of the absorber

material(s) causes the promotion of electrons from the ground state,

approximated by the highest occupied molecular orbital (HOMO), to the excited

state, approximated by the lowest unoccupied molecular orbital (LUMO). These

photoexcitation depends upon the value of the optical absorption coefficient and

on the thickness of the donor material. Then the excitons are generated which,

consists of an electron and a hole paired by an energy that is smaller than the

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respectively). The difference between two energies is called exiton binding

energy which, is around 0.1-0.2 eV in organic materials. The occupation of these

exited states, the LUMO by the electron, and the HOMO by the hole, is termed

as a nonrecombined excitons.

2. Diffusion of the excitons: Excitons produced within a diffusion length from the

D/A interface will have the chance to reach it before decaying, radiatively or not.

Diffusion takes place as long as recombination process do not takes place.

Forster (long range) or Dexter (between the adjacent molecules) transfers can

takes place between an excited molecule.

3. Dissociation of the excitons: If the offsets of the energy levels of the D and the A

materials are higher than the exciton binding energy, excitons dissociate at the

D/A interface. Excitons photogenerated in the donor side will dissociate by

transferring the electron to the LUMO level of the acceptor and retaining the

positive charge, while those created in the other side will transfer the hole to the

HOMO of the donor while retaining the negative charge. This step leads to the

formation of free charge carriers.

4. Charge transport and charge collection: The charge carriers diffuse to the

electrodes through the respective materials (electrons in the acceptor and holes in

the donor). The charges reach the electrodes and are collected. For this to occur

most efficiently, the following conditions must be satisfied:

(EF)cathode < (ELUMO)acceptor and (EF)anode < (ELUMO)donor.

In each of the above steps several phenomena can take place that decrease the

efficiency of the global process, so that only a limited portion of the photons reaching

the cell are able to generate “useful” charge carriers. Thus, the optimization of each step is fundamental to extract as much energy as possible from the device.

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1.2.2 Characterization of Solar Cell Device

Solar cells are further characterised by measuring the current-voltage I(V) curve

under illumination of a light source that mimics the sun spectrum. A typical

current-voltage I(V) curve of a polymer solar cell is shown in Figure 1.4. Since

organic semiconductors show very low intrinsic carrier concentration, the

metal-insulator-metal (MIM) model seems to be best suited to explain this

characteristic. The characteristic points used to characterise a solar cell are labelled in

Figure 1.4. In addition, for each of these points, the energy diagram for a single-layer

cell with an indium tin oxide (ITO) anode and aluminium cathode is displayed.

(a) The current delivered by a solar cell under zero bias is called short circuit current

(Isc). In this case, exciton dissociation and charge transport is driven by the

so-called built-in potential. This can be determined by the product of

photoinduced charge carrier density and the charge carrier mobility within the

organic semiconductors:

Where, n is the density of charge carriers, e is the elementary charge, μ is the

mobility, and E is the electrical field. Therefore, for improving the short circuit

current, high mobility/low band gap materials are essential. In the MIM picture,

this potential is equal to the difference in work function () of the hole- and electron-collecting electrodes. For polymer solar cells, the transparent ITO

electrode is often chosen (ITO = 4.7 eV) in combination with a low work

function material (Ca = 2.87 eV, Mg = 3.66 eV, Al = 4.24 eV) as

counter-electrode to achieve a high internal field.

The external quantum efficiency (EQE) is simply the number of electrons

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(b) The voltage where the current equals zero is called open circuit voltage (Voc). In

the MIM picture this situation is described by the case where the band is flat,

since the applied voltage equals the difference in the work function of the

electrodes. (Note that diffusion effects are neglected in this simplified picture)

(c) When V > Voc, the diode is biased in the forward direction. Electrons are now

injected from the low work function electrode into the LUMO and holes from the

high work function electrode into the HOMO of the organic layer, respectively.

(d) When V < 0, the diode is driven under a reverse biased condition the solar cells

works as a photodiode. The field is higher than in (a) which often leads to

enhanced charge generation and/or collection efficiency.

The point where the electrical power P = I × V reaches the maximum value represents

the condition where the solar cell can deliver its maximum power to an external load.

It is called the maximum power point. The ratio of this maximum electrical power

Pmax to the product of the short circuit current and the open circuit voltage is termed

the fill factor (FF).



Ideally, the fill factor should be unity, but losses due to transport and recombination

result in values between 0.2–0.7 for organic photovoltaic devices.

The photovoltaic power conversion efficiency (η) is then calculated for an incident

light power Pin:

 

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Figure 1.4 Current (voltage) characteristics of a typical organic diode shown together

with the metal-insulator-metal (MIM) picture for the characteristic points. (a) Short

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1.2.3 Organic Photovoltaic Device Architectures

The organic cells reported in the literature can be categorized by their device

architecture as having single layer, bilayer, disordered bulk heterojunction; or ordered

bulk heterojunction structure (Figure 1.5)

Figure 1.5 Four device architectures of conjugated polymer-based photovoltaic cells:

(a) single-layer PV cell; (b) bilayer PV cell; (c) disordered bulk heterojunction; (d)

ordered bulk heterojunction.12

(a) Single-layer PV cell: Although it is possible to generate a built-in field in an

inorganic semiconductor through the controlled placement of n- and p-type

dopant atoms, it is difficult to controllably dope most conjugated polymers. As a

result of this, conjugated polymers are usually made as pure as is practically

possible and can effectively be considered to be intrinsic semiconductors.

Generating built-in electric fields within a film in the dark requires sandwiching

the polymer between electrodes with varying work functions or incorporating

interfaces with a second semiconductor into the device structure.13 In single-layer conjugated polymer PV cells, the sign and magnitude of Voc could at

least be partially attributed to an electrode work-function difference. Although

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typically very low.14

(b) Bilayer PV cell: C. W. Tang in 1985 discovered that, by making two-layer PV

cells with organic semiconductors that have offset energy bands, the external

quantum efficiency of PV cells could be improved to 15% at the wavelength of

maximum absorption.5 The improved efficiency resulted from exciton dissociation at the interface between the two semiconductors. Excitons generated

within a few nanometers of the heterojunction could diffuse to the interface and

undergo forward electron or hole transfer. This process of forward charge

transfer led to the spatial separation of the electron and hole, thereby preventing

direct recombination and allowing the transport of electrons to one electrode and

holes to the other. Because there were essentially no minority free carriers in the

undoped semiconductors, there was little chance of carrier recombination once

the charges moved away from the interface, despite the long transit times to the

electrodes. Sariciftci et al. first applied this two-layer technique to a conjugated

polymer PV cell by evaporating C60 on top of a spin-cast MEH-PPV layer.15

However, in the organic PV cell, the excitons in these materials need to be

generated near the interface for dissociation to occur before recombination. The

exciton diffusion length in several different conjugated polymers has

subsequently been measured to be 4−20 nm.16

Because the exciton diffusion

length in a conjugated polymer is typically less than the absorption length of the

material ( 100 nm), the EQE of a bilayer device made with a conjugated

polymer and another semiconductor is ultimately limited by the number of

photons that can be absorbed within an exciton diffusion length of the interface.

(c) Disordered bulk heterojunction: To address the problem of limited exciton

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independently intermixed two conjugated polymers with offset energy levels so

that all excitons would be formed near an interface. They observed that the

photoluminescence from each of the polymers was quenched. This implied that

the excitons generated on one polymer within the film reached an interface with

the other polymer and dissociated before recombining. This device structure,

called a bulk heterojunction, provided a route by which nearly all photogenerated

excitons in the film could be split into free carriers.

(d) Ordered bulk heterojunction: In all of the bulk heterojunction devices that we

have described above, the conjugated polymer and electron acceptor have been

randomly interspersed throughout the film. The randomly distributed interface

between the two semiconductors can lead to incomplete PL quenching in the

conjugated polymer in regions of the polymer that are more than an exciton

diffusion length away from an acceptor. For these reasons, some have sought to

create well-ordered conjugated polymer−electron acceptor films. In an ideal

device structure, every exciton formed on the conjugated polymer will be within

a diffusion length of an electron acceptor, although quantitative modeling has

pointed out that some light emission will still occur in the polymer even if this is

the case.19 Polymeric bulk heterojunction devices, whose photoactive layer is composed of a blend of bicontinuous and interpenetrating donor and acceptor,

can maximize interfacial area between the donor and the acceptor. In addition,

these devices can be processed in solution, such as spin-coating or roll-to-roll

printing, thereby contributing several attractive advantages such as low-cost,

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1.2.4 Comparison Between Organic and Inorganic Solar Cell

The mechanism underneath the operation of a polymer (or an organic) solar cell

exhibits, of course, many similarities with that of inorganic cells, but also some

distinctions, arising from a few important different characteristics of the materials

involved:

(1) While inorganic semiconductors exhibit a band structure, organic semiconductors

possess discrete energy levels (molecular orbitals). Nevertheless, the term

“bandgap” is often improperly used for organic semiconductors.

(2) In solar cells based on inorganic semiconductors such as silicon, the absorbed

photons lead to the direct creation of free charge carriers. In contrast, in organic

semiconductors based on π-conjugated systems because of the low dielectric constant of these materials, light absorption leads to the creation of excitons,

strongly Coulombically bound electron-hole pairs. In organic heterojunctions, the

driving force for exciton dissociation is provided by the energy offset between the

molecular orbitals of the donor and acceptor. Exciton dissociation into free

charge carriers thus represents a key process that imposes one of the major

limitations to the power conversion efficiency of organic solar cells.

(3) When a bound hole-electron pair (exciton) is generated in an inorganic

semiconductor, its immediate dissociation is observed. Excitons in organic

semiconductors are tightly bound (binding energy of around 0.3−0.5 eV) and

dissociation must be promoted in some way avoiding radiative recombination.

(4) Compared to inorganics, charge carrier mobilities in organic semiconductors are

very low.

(5) Light absorption coefficients of organic materials are much higher than those of

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1.3 Literature Survey of Polymer Solar Cell Materials

In a typical polymeric BHJ PVC, the photoactive blend layer, sandwiched

between an indium tin oxide (ITO) positive electrode and a metal negative electrode,

may be composed of a low band gap conjugated polymer donor and a soluble

nanosized acceptor.9-12 A fullerene derivative, [6,6]- phenyl-C61-butyric acid methyl ester (PCBM), showing better solubility than C60 in common solvents, is a widely

utilized acceptor. As a component in the active layer, a conjugated polymer donor

serves as the main absorber to solar photon flux, as well as the hole transporting

phase.9,20 Thus a low band gap feature to match the solar spectrum (Figure 1.6) and fast hole mobility are basic requirements to design an ideal polymer donor.

Figure 1.6 Absorption coefficients of films of commonly used materials in

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1.3.1 Design Considerations for Low Band Gap Polymers

In recent days, interests in the design and synthesis of conjugated polymers have

been increased for the applications of electronic and photonic devices. It remains a

key challenge to synthesize ideal low band gap (LBG) polymers with high intrinsic

conductivities to develop their potential applications in highly efficient

bulk-heterojunction (BHJ) solar cells.Concerning the band gaps of LBG polymers,

the following factors should be taken into the account: intra-chain charge transfers,

bond-length alternation, aromaticity, substituents effects, intermolecular interactions,

and π-conjugation length etc.22-23

The low band gap copolymers reported are often based on thiophene but other

electron rich aromatic units such as pyrrole are also found. Identical for these

copolymers are the alternation between electron donor (electron rich) and electron

acceptor (electron deficient) units. The high energy level for the HOMO of the donor

and the low energy level for the LUMO of the acceptor results in a lower band gap

due to an intra-chain charge transfer from donor to acceptor.23 Planarity along the aromatic backbone results in a low band gap, due to a high degree of delocalization of

the π-electrons. The alternation between single and double bonds along the polymer

chain has a tendency to increase the band gap. A reduction of the difference in bond

length alternation is achieved by the alternation of donor and acceptor units along the

conjugated polymer chain thus lowering the band gap. As described interactions

between acceptor and donor enhance double bond character between the repeating

units, this stabilizes the quinoid form of e.g. benzo-bis(thiadiazole) (Figure 1.7)

formed within the polymer backbone, and hence a reduction in band gap is

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Figure 1.7 Resonance structures in benzo-bis-thiadiazole.25

If the HOMO level of the donor and the LUMO level of the acceptor are close in

energy it results in a low band gap as shown in (Figure 1.8). Therefore, to achieve a

lower band gap the strength of the donor and the acceptor must be increased. This is

efficiently achieved by using electron withdrawing groups such as CN, NO2,

quinoxalines, pyrazines or thiadiazole on the acceptor and electron donating groups

such as thiophene or pyrrole on the donor.24

Figure 1.8 Alternating donor–acceptor units lower the effective band gap by orbital

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In order to increase the power conversion efficiency (PCE) in BHJ solar cells,

some important characteristics of LBG polymers need to be dealt, such as: (i) a more

favorable overlap of the absorption spectrum of the active layer with the solar

emission26 – Many classes of LBG polymers with the absorption edges extended into the near-infrared regions have been synthesized and investigated.27 (ii) a better charge carrier mobility28 – This can be improved by optimization of intra-chain ordering (co-planarity and conjugation length) and inter-chain stacking, which often

can be increased upon annealing the BHJ solar cell devices.29 (iii) an optimized relative positions of the energy levels of the electron donors and acceptors30 – The maximization of the open circuit voltage (Voc) is correlated to have more efficient

charge separation between electron-donor polymers and electron-acceptor PCBM. For

this purpose, the donor polymer should exhibit a band gap between 1.2 and 1.9 eV,

which corresponds to a HOMO energy level between -5.8 and -5.2 eV and LUMO

energy level between -4.0 and -3.8 eV.31 In order to achieve higher efficiencies of BHJ solar cell devices, the difference of the LUMO levels between donor polymer

and acceptor PCBM needs to be at least 0.3 eV.32 Otherwise, the driving force for charge separation will be decreased, and also Voc will be reduced by raising the

HOMO level of the donor polymer. Therefore, in order to synthesize LBG polymers,

the design rules described above suggest that the optimization of HOMO and LUMO

levels of LBG polymers is the most promising strategy to develop BHJ solar cells

with high efficiencies. However, it is difficult to synthesize the LBG polymers with

all three properties like broad absorption spectra, high carrier mobilities, and

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1.3.2 Polymer Solar Cell Materials

Generally, organic materials having delocalized π electrons, absorbing sunlight, creating photo generated charge carriers and transporting these charge carriers can be

used for fabrication of polymer solar cells. These materials are classified to the

electron donors and the electron acceptors.

Figure1.9 Example of organic semiconductors used in polymer solar cells.33

Most of semiconducting polymers are hole-conductors. This kind of

semiconducting polymers was named as the electron donor polymers. Figure 1.9

shows some representative semiconducting polymers. Four important representatives

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electron acceptor polymers like CN-MEH-PPV, F8TB, and small molecules, C60 and

soluble derivatives of C60 and C70, namely PC60BM and PC70BM, are also shown in

Figure 1.8. Fullerenes are considered to be the best electron acceptors so far. This is

because: (i) ultrafast (50 fs) photo induced charge transfer was happened between the

donor polymers and fullerenes; (ii) fullerenes exhibited high mobility, for example,

PC60BM shown electron mobility up to 1 cm2 V-1 s-1 measured by field effect

transistors; (iii) fullerenes shown a better phase segregation in the blend film.34

Dialkoxy-substituted poly(para-phenylene vinylene)s (PPVs), for example,

poly[2-methoxy-5-(2-ethyl-hexyloxy)- 1,4-phenylene vinylene] (MEH-PPV) and

poly[2-methoxy-5- (3‟,7‟-dimethyloctyloxy)-1,4-phenylene vinylene] (MDMOPPV)

show strong absorption in the visible light band. Notable PCE values of 2-3% have

been reproducibly achieved.35-36

1.3.3 P3HT Containing Polymer Solar Cells

During the last five years, research efforts have focused on

poly(alkyl-thiophenes), and in particular on P3HT. In 2002, the first encouraging

results for P3HT:PCBM solar cells with a weight ratio of 1:3 were published.37 At that time, the short-circuit current density was the largest ever observed in an organic

solar cell (8.7 mA cm-2), and resulted from an EQE that showed a maximum of 76% at 550 nm. Many optimization methods, such as using different solvents to fabricate

the active layer, thermally annealing the active layer or the device, film forming

speed, additives to the active layer, optical spacer, anode or cathode interfacial layer,

and tandem structure, have been extensively carried out with P3HT as donors and

have demonstrated significant improvements in the photovoltaic performance of BHJ

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Figure 1.10 Nonexhaustive survey of reports focusing on photovoltaic devices based

on P3HT:PCBM blends.31

Despite the promising efficiencies obtained for the P3HT:PCBM devices, this

system has several drawbacks: P3HT:PCBM based solar cells generate a low open

circuit voltage of only 0.65−0.70 V. This low open circuit voltage is one of the major loss mechanisms in this system, taking into account that only photons with energies

exceeding the optical band gap of P3HT of 1.9 eV are absorbed. The absorbance

coefficient of PCBM in the visible region is relatively low, such that the

photoresponse of the P3HT:PCBM devices is mainly due to the P3HT absorption

alone. As these devices are limited by their photocurrent generation and intrinsic

absorption properties, further increase of PCE is rather difficult. Therefore, an

alternative approach to get a higher efficiency is to use LBG donor-acceptor

polymeric materials, as their electronic and optoelectronic properties can be tuned

through intra-molecular charge transfers.

1.3.4 Other Low Band-Gap Containing Polymer Solar Cells

Despite the advances of PVC performance with the classic MEH-PPV,

MDMO-PPV, and P3HT as the BHJ donor phase reaching PCE up to 5%, further

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large scale commercialization as a renewable energy source. One of the routes for

such improvements is the design of new polymer donors that have extended

absorption edge to match solar terrestrial radiation, higher carrier mobility, and better

energy alignment with acceptors to reach high open circuit voltage. These studies

have paved the pathway toward better understanding of the nature of BHJ solar cells,

and some of them might be promising candidates for future commercial applications.

One of the most widely used strategies to make narrow band gap donor polymers

is the synthesis of an alternating copolymer from electron-rich (donor) and

electron-deficient (acceptor) units in their backbone. One of the such acceptor

heterocycle, 2,1,3-benzothiadiazole has been utilized to construct some n-type

semiconducting polymers in cooperation with varieties of electron-donating (D) units

to show outstanding photovoltaic performances.27 (Figure 1.11)

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Copolymers of a dialkylated fluorene unit with a donor-acceptor-donor (DAD)

group have been researched quite extensively in the past years. The fluorene unit

provides a group which has a broad energy gap, which is stable and exhibits a high

hole mobility, resulting in high values for Voc and moderate to good fill factors. The

performance of solar cells fabricated with these materials is, depending on the

material and conditions used, moderate to excellent with some cell achieving

efficiencies of 2.8%.48-51 (figure 1.12)

Figure 1.12 General structure of the fluorene copolymers with a variety of

acceptors.General structure of the fluorene copolymers with a variety of acceptors.

The phenothiazine moiety with its electron-rich sulfur and a nitrogen heteroatom

incorporated into the donor-acceptor conjugated polymer can potentially serve as the

donor segment in BHJ solar cells and improve their hole-transporting abilities.52-55 (figure 1.13)

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Figure 1.13 General structure of the phenothiazine containing copolymers.

One of the important developments in the solar cell materials are the conjugated

polymers containing an alkylated carbazole. These polymers can be regarded as a

relative of the family of diaklylated polyfluorene polymers containing a

donor-acceptor-donor group (Figure 1.14). The main reasons for using a carbazole

unit over a fluorene unit is that they are reported to have better hole transporting

properties with respect to the fluorene unit, while maintaining the stability.

Efficiencies up to 5.6% have been reported matching the performance of P3HT based

solar cells leading to Voc values of 0.97 V.56

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Low bandgap diketopyrrolopyrrole based copolymers have been recently

synthesized by alternating electron-rich and electron-deficient units. These polymers

(Figure 1.15) have proved to exhibits excellent ambipolar charge transport properties

(hole and electron mobilities up to 0.1 cm2 V-1 s-1) and the power conversion efficiency (PCE) goes up to 4.0%.57-58

Figure 1.15 Structure of the diketopyrrolopyrrole containing copolymers with a

variety of donors.

LBG polymers containing electron donating moieties from the 2,2‟-bithiophene

unit covalently bridged with an atom, such as C, N, S, and Si at 3,3‟-position, have attracted considerable research attentions. The bridging atoms at 3,3‟-position of donor moieties play an important role for LBG polymers in terms of solubility,

planarity, band gap, and interchain packing, as well as for the performance of the BHJ

solar cells.59 As compared with polythiophene and polyfluorene derivatives, LBG polymers based on these donor moieties showed relatively high conductivities due to

more extensive π-conjugation lengths, narrow band gaps, high planarities, and strong intermolecular π-π interactions of donor units.60

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Figure 1.16 Structure of the polymers containing donating moieties from the

2,2‟-bithiophene unit covalently bridged with an atom at 3,3‟-position.

Following the development of the bulk heterojunction structure, recent years

have seen a dramatic improvement in the efficiency of polymer solar cells.

Maximizing the open-circuit voltage in a low-bandgap polymer is one of the critical

factors towards enabling high-efficiency solar cells. Study of the relation between

open-circuit voltage and the energy levels of the donor/acceptor in bulk

heterojunction polymer solar cells has stimulated interest in modifying the

open-circuit voltage by tuning the energy levels of polymers. Recently, by tuning the

open-circuit voltage of polymer solar cells based on the structure of a low-bandgap

polymer, PBDTTT (Figure 1.17), yielded power conversion efficiency as high as

7.7%, as certified by the National Renewable Energy Laboratory.61

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1.3.5 Conjugated Polyelectrolytes for Solar Cells

Motivations for examining the potential incorporation of conjugated

polyelectrolytes into solar cell development include their easy processability, their

ability to be used in layer by layer (LBL) processing, and the fact that their

applications in a variety of chemical and sensory schemes have shown that they are

efficiently quenched by electron acceptors. Due to the success in solar cells by

increasing the donor/ acceptor interfacial area, it is not surprising that the LBL

approach, and its multilayer heterostructure, has been of great interest. Some of such

conjugated polyelectrolytes utilized in the polymer solar cells are depicted in figure

1.18

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1.3.6 Characterization of Active Materials for Polymer Solar Cells.

The molecular design of D/A pairs for high efficiency PSC has to meet a lot of

optoelectronic requirements, other than an excellent processability from solution, very

high chemical purity, etc. To this end, an extensive characterization of the newly

synthesized materials is required, involving multidisciplinary expertise (Figure 1.19),

to assess their potentials as promising donors or acceptors for polymer solar cells.

Chemists are making a great effort in the direction of energy level engineering and a

variety of fullerene derivatives and p-type conjugated polymers (vide infra) have been

proposed as functional materials toward high efficiency PSC.

Figure 1.19 Nonexhaustive list of investigation techniques required for an extended

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1.4 Motivation

The main objective of this dissertation is to construct the donor-acceptor polymer

or metallo-polymer architectures by incorporating various donors and acceptors and to

study their performance in polymer bulk heterojunction solar cells as electron donors

with fullerene derivatives as electron acceptors. Addition of electron-withdrawing

imine nitrogen to a conjugated polymer backbone generally enhances its

electron-accepting properties and makes it susceptible to n-doping. Benzodiazole units

are, in that sense, typical examples of such units containing imine nitrogen,56 which have been widely used electron acceptors for the synthesis of D-A polymers. For

example, copolymers of benzodiazole with variety of donors such as, fluorene,49 silafluorene,86 carbazole,56 dithienosilole,87 dithienocyclopentadiene,26 and dithieno[3,2-b:2‟,3‟-d]pyrroles88 were synthesized and applied to PSCs, yielding PCE values in the range of 0.18-5.4%. Recently, many LBG copolymers have been

synthesized by sandwiching acceptors in the midst of two thiophene units to alleviate

the severe steric hindrance between the electron donors and acceptors, resulting in

more planar structures to facilitate inter-chain associations and improve the hole

mobilities of the LBG polymers. Among all heterocyclic donors, phenothiazine

contains both electron-rich sulfur and nitrogen heteroatoms. The electron-rich nature

of phenothiazine contributes for the efficient electron donor and hole transporting

materialsin polymers. In order to have better photophysical, electrochemical, and

photovoltaic properties in the polymers, we incorporated of phenothiazine donor units

with various benzodiazole acceptors (such as benzothiadiazole, benzoselenodiazole,

and benzoxadiazole units) sandwiched between two hexyl thiophene units to form

alternating conjugated donor-acceptor polymers. These polymers were synthesized by

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strengths on the electronic and optoelectronic properties of the LBG polymers were

also investigated. In addition, the PSC devices fabricated by polymer/PC61BM or

polymer/PC71BM blends sandwiched between a transparent anode (ITO/PEDOT:PSS)

and a cathode (Ca) were explored.

LBG polymers containing electron donating moieties from 2,2‟-bithiophene unit

covalently bridged with an atom, such as C, N, S, and Si, at 3,3‟-position have

attracted considerable research attentions. The bridging atoms at 3,3‟-position of donor moieties play an important role for LBG polymers in terms of solubility,

planarity, band gap, and interchain packing, as well as for the performance of the bulk

heterojunction (BHJ) solar cells.59 As compared with polythiophene and polyfluorene derivatives, LBG polymers based on these donor moieties showed relatively high

conductivities due to more extensive π-conjugation lengths, narrow band gaps, high planarities, and strong intermolecular π-π interactions of donor units.105-106

Again, To

obtain the broad absorption bands with high absorptivities, electron-donating groups

and/or electron-withdrawing groups are substituted on the main-chains of the

conjugated polymers to raise the HOMO levels and/or to reduce the LUMO levels of

the polymers.107-108 Hence, introduction of electron-withdrawing cyano-vinylene groups to polymer backbones to lower the LUMO levels,109 tune their electro-optical properties,110 and enhance the electrochemical stabilities of the polymers111 are desirable for optoelectronic device applications. Moreover, LBG polymers containing

electron-accepting cyano-vinylene groups were proven to possess higher hole

mobilities,112 and were applied as photovoltaic materials in BHJ solar cells.113-116 However, the PCE values of these photovoltaic cells are still low at present. All these

research results inspire further development in exploring cyclopentadithiophene- and

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electrochemical, and photovoltaic properties. Based on this concept, soluble

cyclopentadithiophene- and dithienosilole-based LBG D-A polymers (CPDT-CN,

DTS-CN) containing -cyano-thiophenevinylene groups are designed and

synthesized. The effects of the bridged atoms on the optical, electrochemical, charge

transporting and photovoltaic properties of the polymers are compared and reported

also in this study.

Nowadays, some terpyridyl Ru(II) complexes have attracted researchers to

use in the applications of photovoltaic cells (PVC).62-65,147-150 The insertion of ruthenium metals into conjugated backbones has several advantages, such as to

facilitate the charge generation by extending its absorption range due to its

characteristic long-lived metal to ligand charge transfer (MLCT) transition136 and to exhibit a reversible RuII,III redox process along with some ligand-centered redox processes. Motivations for examining the potential incorporation of such

conjugated polyelectrolytes into solar cell development include the easy

processability, layer-by-layer (LBL) processing capability, and also due to

efficiently quenched by electron acceptors. But PCE values of these devices were

limited either by the low open-circuit voltage (Voc) or low short circuit current (Jsc).

Due to relatively high HOMO levels and less sensitization ranges in all reported

polymers, there were inefficient photocurrents generated which probably affected

their PCE values. One of the feasible solutions to conquer these problems, i.e. to

get a higher Voc value and a more favorable overlap of the absorption spectra from

both active layer and solar emission, is to introduce electron donor-acceptor

structures to the cores of bis-terpyridyl ligands. The incorporation of the thiophene

donor units with benzodiazole acceptor units at the cores of bis-terpyridyl ligands

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and photovoltaic properties are very intriguing us. So, we design, synthesis,

properties, and device applications of RuII-containg metallo-polymers containing donor-acceptor (D-A) bis-terpyridyl ligands bearing different benzodiazole

acceptors, including benzothiadiazole, benzoselenodiazole, and benzoxadiazole

cores sandwiched between symmetrical thiophene and terpyridyl units. The effects

of their donor-acceptor strengths on the electronic and optoelectronic properties

were also investigated. In addition, the PVC devices fabricated by these

bis-terpyridyl ligands and metallo-polymers with [6,6]-phenyl-C61-butyric acid

methyl ester (PCBM) inserted between a transparent anode (ITO/PEDOT:PSS) and

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Chapter 2. Synthesis and Applications of Low-Bandgap Conjugated Polymers

Containing Phenothiazine Donor and Various Benzodiazole

Acceptors for Polymer Solar Cells

2.1 Introduction

In spite of poor long-term stability, polymer solar cell (PSC) devices based on

conjugated polymers as electron donors and fullerene derivatives as electron acceptors

are of broad interests because of the advantages of low cost, light-weight flexible

devices, tunable electronic properties, and ease of processing for the conversion of

solar energy to electricity.9-10,21,69 Although poly(3-hexylthiophene) (P3HT) is proven to be one of the most efficient donor materials ever tested in PSCs for giving the

power conversion efficiency (PCE) up to 5%,37-47 further enhanced PCE values are limited due to both lower photocurrent generation and intrinsic absorption properties.

In order to conquer these problems, low-bandgap (LBG) polymers composed of

electron-rich (donor) and electron-deficient (acceptor) units have been utilized

recently in PSCs with fullerene derivatives, such as [6,6]-phenyl-C61-butyric acid

methyl ester (PC61BM) or [6, 6]-phenyl-C71-butyric acid methyl ester (PC71BM),

yielding a power conversion efficiency (PCE) value up to 7.7%.61,70-74 Polymer solar cells consisting of such donor-acceptor (D-A) LBG polymers have attracted more

attention owing to their tunable optical, electrochemical, electronic, and photovoltaic

properties.23-24 Incorporation of wide ranges of donors and acceptors into LBG polymers can manipulate the electronic structures, i.e., the highest occupied molecular

orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) levels through the

partial intramolecular charge transfer (ICT) in the D-A systems.75-76 By optimizing materials and device structures, photovoltaic parameters, such as the short-circuit

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current (Jsc) and open-circuit voltage (Voc), can be further improved to obtain higher

PCE values in the PSCs. In solar cell devices, Jsc is determined by the creation and

subsequent dissociation of excitons at the polymer/acceptor interface followed by

transport of free charge carriers towards the collecting electrodes,77 Voc is primarily

determined by the effective band gap of the bulk hetero-junction (BHJ) film.78 For this purpose, the electron donor polymer should exhibit a band gap between 1.2 and

1.9 eV, which corresponds to a HOMO energy level between -5.8 and -5.2 eV and a

LUMO energy level between -4.0 and -3.8 eV.31 Again, if the energy difference between the LUMO levels of polymer and acceptor is less than 0.3 eV,32 the driving force for charge separation will be reduced, and Voc can be reduced by raising the

HOMO level. Consequently, it is of great importance to match the energy levels of the

polymer and acceptor carefully to develop BHJ solar cells with high efficiencies.

Among all heterocyclic compounds, phenothiazine contains both electron-rich

sulfur and nitrogen heteroatoms. The electron-rich nature of phenothiazine contributes

for the efficient electron donor and hole transporting materials in polymers and

organic molecules for photo-induced charge separation and it has been also proven as

a superior electron donor for reductive quenching.79 Due to their unique electro-optical properties, these materials are potential candidates for diverse

applications for light-emitting diodes,80-81 solar cells, chemiluminescence devices,82-83 and organic field effect transistors.84 Phenothiazine ring hampers stacking aggregation and intermolecular excimer formation in the main chain of the polymer due to its

non-planar structure.85 However, till now only a limited number of phenothiazine-based polymers for photovoltaic devices have been explored.52-55

Addition of electron-withdrawing imine nitrogen to a conjugated polymer

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to n-doping (reduction). Benzodiazole units are, in that sense, typical examples of such

units containing imine nitrogen.56 2,1,3-Benzothiadiazole is a widely used electron acceptor for the synthesis of D-A polymers. For example, copolymers of

benzothiadiazole with fluorene,49 silafluorene,86 carbazole,56 dithienosilole,87 dithienocyclopentadiene,26 and dithieno[3,2-b:2‟,3‟-d]pyrroles88 were synthesized and applied to PSCs, yielding PCE values in the range of 0.18-5.4%. Recently, many

photovoltaic papers have reported LBG copolymers made of electron donors and

acceptors sandwiched between two thiophene units.26,49,56,87-88 Incorporation of acceptor units in the midst of two thiophene units, alleviate the severe steric hindrance

between the electron donors and acceptors, resulting in more planar structures to

facilitate inter-chain associations and improve the hole mobilities of the LBG

polymers. Despite of these advantages, addition of thiophene units could induce

solubility problems and yield low molecular weights in polymers.89 To utilize the aforementioned merits of thiophene units, structural modifications, such as

incorporation of alkyl or alkoxy chains on the 3- and/or 4-position of thienyl units90 or addition of supplementary alkylated thiophene units,91 have been outfitted to acquire higher molecular weights and better solubilities than the original polymers without

any soluble side-chains.

In order to have better photophysical, electrochemical, and photovoltaic

properties in the resulting LBG polymers, the incorporation of phenothiazine donor

units with various acceptor units are very intriguing and thus to motivate this study.

Herein, we report the design, synthesis, properties, and device applications of

phenothiazine-based alternating conjugated donor-acceptor polymers, in which the

acceptor benzodiazole units include benzothiadiazole, benzoselenodiazole, and

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synthesized by palladium(0)-catalysed Suzuki coupling reactions. The effects of

donor-acceptor strengths on the electronic and optoelectronic properties of the LBG

polymers were also investigated. In addition, the PSC devices fabricated by

polymer/PC61BM or polymer/PC71BM blends sandwiched between a transparent

anode (ITO/PEDOT:PSS) and a cathode (Ca) were explored.

2.2 Experimental Section 2.2.1 Materials

All chemicals and solvents were reagent grades and purchased from Aldrich,

ACROS, Fluka, TCI, TEDIA, and Lancaster Chemical Co. Toluene, tetrahydrofuran,

and diethyl ether were distilled over sodium/benzophenone to keep anhydrous before

use. Chloroform (CHCl3) was purified by refluxing with calcium hydride and then

distilled. If not otherwise specified, the other solvents were degassed by nitrogen 1 h

prior to use.

2.2.2 Measurements and Characterization

1

H NMR and 13C NMR spectra were recorded on a Varian Unity 300 MHz spectrometer using CDCl3 solvent. Elemental analyses were performed on a

HERAEUS CHN-OS RAPID elemental analyzer. Thermogravimetric Analyses

(TGA) were conducted with a TA Instruments Q500 at a heating rate of 10°C/min under nitrogen. The molecular weights of polymers were measured by gel permeation

chromatography (GPC) using Waters 1515 separation module (concentration = 1

mg/mL in THF; flow rate = 1 mL/min), and polystyrene was used as a standard with

THF as an eluant. UV-visible absorption spectra were recorded in dilute

chlorobenzene solutions (10-5 M) as well as on solid films (spin-coated with a spin rate ca. 1000 rpm for 60 s on glass substrates from chlorobenzene solutions with a